Endosomal sorting of GLUT4 and Gap1 is conserved between yeast and insulin-sensitive cells

Abstract

SummaryThe insulin-regulated trafficking of the facilitative glucose transporter GLUT4 in human fat and muscle cells and the nitrogen-regulated trafficking of the general amino acid permease Gap1 in the yeast Saccharomyces cerevisiae share several common features: Both Gap1 and GLUT4 are nutrient transporters that are mobilised to the cell surface from an intracellular store in response to an environmental cue; both are polytopic membrane proteins harbouring amino acid targeting motifs in their C-terminal tails that are required for their regulated trafficking; ubiquitylation of both Gap1 and GLUT4 plays an important role in their regulated trafficking, as do the ubiquitinbinding GGA (Golgi-localised, c-ear-containing, ARF-binding) adaptor proteins. Here, we find that when expressed heterologously in yeast, human GLUT4 is subject to nitrogen-regulated trafficking in an ubiquitin-dependent manner similar to Gap1. In addition, by expressing a GLUT4/Gap1 chimeric protein in adipocytes we show that the carboxy-tail of Gap1 directs intracellular sequestration and insulin-regulated trafficking in adipocytes. These findings demonstrate that the trafficking signals and their cognate molecular regulatory machinery that mediate regulated exocytosis of membrane proteins are conserved across evolution.

The deubiquitinating enzyme USP25 binds tankyrase and regulates trafficking of the facilitative glucose transporter GLUT4 in adipocytes

“…This is an attractive model given that IRAP and TNKS are required for the formation of GSVs and insulin-regulated trafficking of GLUT4, but GLUT4 is not required for insulin-regulated trafficking of IRAP 10 , 11 . Further support for this novel hypothesis comes from our work using the yeast Saccharomyces cervisiae as a model system to study GLUT4 trafficking 5 , 17 . While expression of human GLUT4 in yeast results in its ubiquitination as in mammalian cells, in yeast this modification results in vacuolar degradation of the transporter 5 , 17 .…”

“…Further support for this novel hypothesis comes from our work using the yeast Saccharomyces cervisiae as a model system to study GLUT4 trafficking 5 , 17 . While expression of human GLUT4 in yeast results in its ubiquitination as in mammalian cells, in yeast this modification results in vacuolar degradation of the transporter 5 , 17 . These data suggest that while some aspects of regulated trafficking through the endosomal system are conserved through evolution, including ubiquitination and GGA-dependent sorting, yeast lack some of the molecular mechanisms that exist in insulin-responsive cells to localise GLUT4 to insulin-responsive GSVs 5 , 17 .…”

A proteolytic pathway that controls glucose uptake in fat and muscle

“…This effect may regulate p97 ATPase activity at or near the ERES/ERGIC, and control the targeting of ubiquitylated cargos in endosomal and autophagy pathways [109-111]. Possibly, the ubiquitin-dependent sorting of GLUT4 may involve this mechanism [112]. …”

“…Impaired, insulin-stimulated GSV translocation may therefore contribute to multiple aspects of the metabolic syndrome. As well, GSV-like vesicles are present in a range of differentiated cell types, and regulated exocytic translocation is likely a conserved mechanism that can respond to a variety of extracellular stimuli [25,112,134]. For example, the translocation of AQP2 water channels in the renal collecting duct, and of H + -pumps in gastric parietal cells, may employ a similar vesicle trafficking mechanism.…”

Zinc Finger Protein 407 (ZFP407) Regulates Insulin-stimulated Glucose Uptake and Glucose Transporter 4 (Glut4) mRNA

“…The mechanisms regulating GLUT4 function are highly evolutionarily conserved (7,8), suggesting that experiments utilizing model organisms represent a powerful approach for studying GLUT4 function. We therefore utilized a prior genome-wide genetic screen in Caenorhabditis elegans to guide an siRNA screen in mammalian adipocytes to identify regulators of insulin-stimulated glucose uptake and GLUT4 function.…”